Both natural and synthetic elastomers usually require the use of processing aids to assist mechanical breakdown. Materials such as mixtures of oil soluble sulfonic acids of high molecular weight with a high boiling alcohol and a paraffin oil or a blend of a sulfonated petroleum product and selected mineral oils are presently used as processing aids. Some chemicals used primarily for other purposes have a plasticizing action on rubbers in which they are compounded, i.e. benzylthiazole disulfide, also known as Altax.TM..
Petroleum, paraffinic and vegetable oils, as well as coal tar and petroleum residues or pitches and naturally occurring or synthetic resins have also been used as compounding materials.
Beneficial effects of processing aids carry on through the mixing cycle permitting incorporation of fillers and other ingredients with low power consumption. These materials also reduce internal friction in calendering and extrusion, thus minimizing scorch.
Various types of rosin acids have been used as extenders for high molecular weight SBR. Properties of GR-S Extended With Rosin Type Acids, L. H. Howland, J. A. Reynolds, and R. L. Provost, Industrial and Engineering Chemistry, Vol. 45, No. 5, May 1953. Also included in these initial studies were several nonrosin acids which included tallow fatty acid, oleic acid and naphthenic acid. Reasonably good cured physical properties can be obtained with the rosin type acids, whereas relatively poor physical properties are obtained with the nonrosin acids. Problems associated with the use of rosin acids are cure retardation, high tack and poor low temperature performance, which limit their use as an extender in rubber formulations.
British Patent No. 962,519 describes elastomeric hydrocarbon copolymers of at least one .alpha.-monoolefin and at least one nonconjugated diene which are extended with specific petroleum oils to give normally solid, sulfur curable mixtures.
U.S. Pat. No. 3,951,901 describes a process for preparing a rubber wherein an extending oil with a specific viscosity and a certain specific gravity is added to the copolymer at a particular temperature with a specific agitation so as to form a homogeneous liquid mixture substantially free of particulate copolymer.
U.S. Pat. No. 3,985,701 discloses an oil containing rubber prepared by mixing a rubber selected from the group consisting of natural rubber, homopolymers of conjugated diolefins and copolymers of conjugated diolefins with ethylenically unsaturated monomers, with mineral oil having at least 10 weight percent aromatic compound which is obtained through a specific chemical process.
U.S. Pat. No. 3,687,889 discloses an oil-extended polyisoprene composition which consists of a synthetic polyisoprene and from 15 to 50 parts by weight of a rubber extending oil and a process for producing said mixture.
None of the prior art suggests or discloses the use of the esters, specifically methyl esters, of wood rosins as a total or partial replacement for conventionally accepted extending oils. Further, the prior art does not suggest or disclose the advantageous properties that can be obtained through use of esters of rosin as a replacement for petroleum based extending oils. The unexpected properties obtainable through use of the present invention include increased abrasion resistance (particularly after aging) and lack of extractability from aged cured compounds.
Rosin is a solid resinous material that occurs naturally in pine trees. There are three major sources of rosin, (1) from the oleoresin exudate of the living pin tree, (2) from the oleoresin contained in the aged stump of the longleaf pine; and (3) from the tall oil produced as a by-product in the paper industry.
The aged virgin pine stump is the source of wood rosin. The stump is allowed to remain in the ground for about ten years so that its bark and sapwood may decay and slough off to leave the heartwood rich in resin.
Rosin is a complex mixture of many resin acids and a small amount of nonacidic components. Chemical treatment of rosins, such as hydrogenation, dehydrogenation, or polymerization are known which produce modified resins.
Rosin acids are monocarboxylic acids having the typical molecular formula, C.sub.20 H.sub.30 O.sub.2. Over the years nomenclature of individual acids has changed. In addition to trivial names, such as abietic, levopimaric, etc. three different numbering systems have been used. IUPAC nomenclature names resin acids as derivatives of abietane. The following is a structural formula for abietic acid: ##STR1## Wherein the spacial relationship of substituents on asymmetric carbon atoms are designated as .alpha. or .beta. to denote whether the substituents are above or below the plane of the paper. For example, .alpha.-methyl denotes the methyl group as below the plane and is represented by a dotted line, while .beta.-methyl would be above the plane and is represented by a solid line.
The resin acid molecule possesses two chemically reactive centers, the double bonds and the carboxyl group. Through these, many modifications in structure and numerous derivatives are obtainable. Because rosin is composed of a number of resin acids, the chemistry of its reactions is relatively complex. For example, resin acids react at the double bond in isomerization reactions and reactions with maleic anhydride. Reactions of certain resin acids with hypochlorite have been reported. Hydrogenation and dehydrogenation have also been reported as a means for modifying rosins to render them less susceptible to oxidation by atmospheric oxygen.
In addition to the double bond reactions, rosin acids also undergo typical carboxyl group reactions. Salt and esters of rosin are important commercial derivatives of rosin. Other reactions involve the reduction of the carboxyl group to the alcohol and the conversion of the carboxyl group to the nitrile.
The structurally hindered nature of the resin acid carboxyl group makes it necessary to use high temperatures or generally drastic conditions to bring about esterification. This hindrance is in turn responsible for the unusual resistance of the ester linkage to cleavage by water, acid or alkali. Ester gum is used extensively in cellulose ester lacquers for interior application because of its low cost, solubility in lacquer solvents and the desirable properties it imparts to the lacquer.
The present invention is concerned with the use of esters of rosin acid as a replacement for petroleum based extender oils in rubber compounds, more specifically, tire compounds. The use of methyl esters of rosin acid has unexpectedly improved low temperature performance and provided less tack when compared with rosin acid and also has a significant effect on the abrasion resistance of the compounded rubber. It was also discovered that aged rubber compounds which contained the methyl ester of rosin as the extender had less extractables than similar compounds containing petroleum based extending oils.